The present invention relates to means for effectively removing Volatile Organic Compounds (VOC""s) from various industrial gases, and more particularly to a method and apparatus to facilitate the removal of such compounds trapped on or in adsorbent materials that have previously been utilized for the removal of such VOC""s from a contaminated air stream. More specifically, the present invention relates to a novel method and apparatus for heating such adsorbent materials to effect the removal of adsorbed VOC""s to permit the reuse of such adsorbent material; further, the invention is directed to a method and apparatus that may be utilized to reactivate adsorbent material when such material is an activated carbonaceous material.
Many industrial processes give rise to exhaust gases which contain volatile organic compounds (VOC""s). VOC""s are of concern, due not only to their direct toxicity in the vapor phase, but also because they can be photochemically reactive and lead to ground level ozone increases and increases in other gas phase irritants.
Reducing VOC""s at the source is sometimes an option, but there are many processes for which this approach is not practical. As a result, there have been a number of xe2x80x9cend of pipexe2x80x9d treatment technologies developed to collect or destroy the VOC""s from process exhausts. Early technologies were widely based on thermal oxidation (incineration). So called xe2x80x9coxidationxe2x80x9d equipment is effective at destruction of VOC""s, but has several inherent drawbacks. These include high operating costs, high fuel consumption, and high levels of secondary pollutant generation. Improvement came in the form of rotor concentrator systems. These units can reduce the final treatment volume to a range of 10% to 20% of the original process effluent volume. This is an advantage relative to direct oxidation, but still requires the use of oxidation as final treatment. Therefore, some of the same issues still apply.
More recently, fluidized bed collection systems have shown promise as an improvement over both direct oxidation and rotor systems. The fluid bed is capable of very high percent reductions in process effluent volume to be treated, with a correspondingly high increase in concentration of the VOC""s. In essence, the adsorbent used in the fluid bed system captures the VOC""s in an adsorption section, and releases them at a much higher concentration in a regeneration or desorption section. In this way, the volume of material for final treatment can be reduced by factors of several thousand. This allows for economic destruction of the VOC""s, or recovery of VOC""s in original liquid form for reuse, where practical. Descriptions of fluidized bed systems can be found in the prior art. U.S. Pat. No. 5,904,750 is an example of this type of system for VOC control.
There are also descriptions in prior art of techniques for heating the adsorbents for drying and reactivation in a stand alone device (see for example U.S. Pat. No. 5,089,457). The use of electricity, directly applied to a column of activated carbon, for the purpose of heating is described. The system described in the U.S. Pat. No. 5,089,457 patent comprises a single columnar device, operated in a batch mode, or pulse mode. The purpose is singular, in that the device is used to reactivate spent carbon from a variety of remote industrial sources. It is not described as part of an integrated VOC control system.
A limitation of the device described in U.S. Pat. No. 5,089,457, and others of its kind, is that it does not solve the intrinsic problems associated with the heating of carbon in a columnar configuration, using electricity applied to the carbon itself. Those skilled in the art are aware that the application of electricity to activated carbon, and the heating which results, renders the method prone to the formation of localized hot spots. When contained in a column, activated carbon will display local variations in packing density, and particle-to-particle contact area. When electricity is applied, uneven heating occurs, due to the unpredictable and variable nature of the aforementioned items. As pointed out in the ""457 patent, the radius of the columnar vessel must be limited to seventy-five particle diameters or less; otherwise, the system cannot maintain temperature control, and the intended function of the entire device is lost. This problem makes scale up to more practically sized systems impossible. In practice, even a columnar device with the seventy-five particle diameter limit of the radius suffers from localized heating problems. The most notable aspect of the problem is the large temperature differential which exists from the center for the column of adsorbent to the side. The difference becomes more severe as the diameter of the heating device increases, and as the temperature becomes higher.
It is therefore an object of the present invention to provide a method and apparatus for minimizing the temperature control problems associated with direct electric heating of activated carbon, and other conductive materials, in a columnar devicexe2x80x94regardless of diameter.
It is a further object of the present invention to provide a continuously operating columnar device which utilizes electric current flowing through an adsorbent to achieve both regeneration and reactivation in the same vessel, using either steam or inert gas as appropriate.
It is yet another object of the present invention to provide an integrated VOC control method and apparatus which incorporates regeneration and reactivation techniques into a fully integrated VOC control system.
Another object of the present invention is to provide a method and apparatus for controlling the temperature of an adsorbent mass in a vessel by passive conductive elements strategically placed in a columnar heating section which redistribute current at selected locations to assure even heating of the adsorbent.
The present invention incorporates a method and apparatus for heating adsorbent materials through the conduction of electrical current therethrough by using spaced electrodes positioned within a column of the adsorbent material within a vessel. In one embodiment, the size of the electrodes is varied such that the second electrode incorporates a cross-sectional area of about 50% of the cross-sectional area of the first electrode; as a result of the size differential, the current flowing through the adsorbent material column incorporates a substantial radial component that effectively establishes a relative uniformity of the current density through the column and thus establishes uniformity of the temperature resulting from the heating of the material by the current flow. In an alternative embodiment, the electrodes remain the same size, but a passive conductive element, or a xe2x80x9cdummy electrodexe2x80x9d is positioned between the active electrodes such that the electrical current carrying capacity of this passive electrode alters the current density along the cross-section of the material column. As a result, the current density is rendered more uniform and the temperature of the material across a transverse section of the column is rendered more uniform. The present invention eliminates the radial temperature variations resulting from current heating of the adsorbent material in a vessel that, as described above, is present in prior art systems attempting to heat the adsorbent material by the application of a potential across a column of the material to induce an electrical current therethrough and thus heat the material.